The present invention relates to a manufacturing method for tires, more specifically a method for improving the uniformity of a tire by reducing the after cure radial force variation. In a tire, and more precisely, a radial tire, the after cure radial force variation (RFV) can be affected by many variables introduced from the process of assembly of the green (uncured) tire and during curing of the tire. When the radial force variation in a cured tire exceeds acceptable limits, the result may be unwanted vibrations affecting the ride and handling of the vehicle. For these reasons, tire manufacturers strive to minimize the level of radial force variation in the tires delivered to their customers.
A well-known and commonly practiced method to improve the after cure RFV is to grind the tread surface of the tire in the zones corresponding to excess radial force. This method is effective, but has the drawback of creating an undesirable surface appearance and of removing wearable tread rubber from the product. In addition, this method requires an extra manufacturing step and uses expensive equipment. Alternatively, the after cure RFV may be improved by the method described in U.S. Pat. No. 5,365,781 where the sidewalls of the cured tire are physically deformed in a controlled manner in response to a measured uniformity characteristic. This method eliminates the undesirable removal of tread rubber, but still requires an extra manufacturing step and high-cost equipment.
An alternative to after cure correction of RFV is to treat the sources of RFV in the tire before cure. For example, it is well known in the tire industry to stagger the starting points of the various tire products during the assembly process, followed by observing the effect on after cure RFV. These data are then used to specify an optimum arrangement of product start points for each of the tire building steps according to the configuration that best minimizes after cure RFV. Another approach is disclosed in U.S. Pat. No. 5,882,452 where the before cure radial runout (RRO) of the tire is measured, followed by a process of clamping and reshaping the uncured tire to a more circular form.
Still another approach to a manufacturing method for improved uniformity involves a method where the factors relating to tire building and tire curing that contribute to after cure RRO or RFV are offset relative to a measured before cure RRO. An example of a typical method is given in Japanese Patent Application JP-1-145135. In these methods a sample group of tires, usually four, are placed in a given curing mold with each tire rotated an equal angular increment. The angular increment is measured between a reference location on the tire, such as a product joint, relative to a fixed location on the curing mold. Next, the tires are vulcanized and their composite RFV waveforms recorded. The term “composite waveform” means the raw waveform as recorded from the measuring device. The waveforms are then averaged by superposition of each of the recorded waveforms upon the others. Superposition is a point by point averaging of the recorded waveforms accomplished by overlaying the measured composite waveform from each tire. The effects of the vulcanization are assumed to cancel, leaving only a “formation” factor related to the building of the tire. In like manner, another set of sample tires is vulcanized in a curing mold and their respective RFV waveforms are obtained. The respective waveforms are again averaged by superposition, this time with the staring points of the waveforms offset by the respective angular increments for each tire. In this manner, the effects of tire building are assumed to cancel, leaving only a “vulcanization factor.” Finally, the average waveforms corresponding to the formation factor and the vulcanization factor are superimposed. The superimposed waveforms are offset relative to each other in an attempt to align the respective maximum of one waveform with the minimum of the other waveform. The angular offset thus determined is then transposed to the curing mold. When uncured tires arrive at the mold, each tire is then placed in the mold at the predetermined offset angle. In this manner, the formation and vulcanization contributions to after cure RFV are said to be minimized. A major drawback to this method is its assumption that the formation and vulcanization contributions to after cure RFV are equivalent for each tire. In particular, the factors contributing to the formation factor can vary considerably during a manufacturing run. In fact, these methods contain contradictory assumptions. The methodology used to determine the vulcanization factor relies on an assumption that the step of rotation of the tires in the curing mold cancels the tire building (or formation) effects. This assumption is valid only when the contribution of before cure RRO is consistent from one tire to the next tire, without random contributions. If this assumption is true, then the subsequent method for determination of the formation factor will produce a trivial result.
Further improvements have been proposed in Japanese Patent Application JP-6-182903 and in U.S. Pat. No. 6,514,441. In these references, methods similar to those discussed above are used to determine formation and vulcanization factor waveforms. However, these methods add to these factors an approximate contribution of the before cure RRO to the after cure RFV. The two methods treat the measured before cure RRO somewhat differently. In the method disclosed in reference JP-6-198203 optimizes RRO effects whereas the method disclosed in U.S. Pat. No. 6,514,441 estimates RFV effects by application of a constant stiffness scaling factor to the RRO waveform to estimate an effective RFV. Both these methods continue to rely on the previously described process of overlapping or superpositioning of the respective waveforms in an attempt to optimize after cure RFV.
The most important shortcoming of all the above methods is their reliance of superpositioning or overlapping of the respective waveforms. It is well known in the tire industry that the vehicle response to non-uniformity of RFV is more significant in the lower order harmonics, for example harmonics one through five. Since, the above methods use composite waveforms including all harmonics, these methods fail to optimize the RFV harmonics to which the vehicle is most sensitive. In addition, a method that attempts to optimize uniformity using the composite waveforms can be shown, in some instances, to produce after-cure RFV that actually increases the contribution of the important lower order harmonics. In this instance, the tire can cause more vehicle vibration problems than if the process were not optimized at all. Therefore, a manufacturing method that can optimize specific harmonics and that is free of the aforementioned assumptions for determining the effects of tire formation and tire vulcanization would be capable of producing tires of consistently improved uniformity.